The present invention relates to an echo cancellation circuit and method.
Data echo cancellers have received considerable attention in recent years in connection with digital subscriber loop modems and full-duplex voiceband data modems. With such modems, an inherent two-wire transmission facility is turned into an equivalent four-wire connection by using a hybrid circuit at each end. Data can then be transmitted simultaneously in both directions. However, the attenuation of the hybrid between its two four-wire inputs can be as low as approximately 10 dB. The purpose of an echo canceller is to remove the "near-end cross-talk" or "echo" signal which feeds through the hybrid into the local receiver, interfering with the data signal coming from a distant transmitter. Since the latter transmitted data signal may be highly attenuated (40 to 50 db), the required attenuation of the echo signal is consequently large (on the order to 50 to 60 db) in order to achieve an acceptable signal-to-echo interference ratio at the receiver input for the maximum expected line attenuation.
Most prior art echo canceller implementations completely neglect the effect of non-linear distortion in the echo path or in the echo replica. An exception to this uses an echo canceller with 2.sup.N taps to synthesize an impulse response of N samples, as described in more detail in IEEE Transactions on Communications, Vol. COM-29, 11, Nov. 1981 entitled "A New Digital Echo Canceller for Two-Wire Subscriber Lines." In this canceller, called a "memory compensation" or "table look-up" canceller, the echo canceller assigns an independent output to each possible combination of N transmitted bits, and thus is completely general as to the kind of nonlinearity that it can correct. The price paid for this generality is 2.sup.N taps rather than N, and requires a structure in which at each sample time only one tap weight can be updated. The consequences of this are that for large N the required memory becomes very large and the adaptation very slow.
Achieving a 50 to 60 dB cancellation in a monolithic echo canceller is further complicated by the inherent nonlinearities in monolithic A/D and D/A converters due to processing variations and component variations. These systems have to deal with small amounts of non-linear distortion, i.e., a channel which is "almost" linear. It would therefore be highly desirable to provide an improved echo cancellation circuit and method which can correct for small amounts of non-linear distortion without a large complexity penalty or adaptation speed penalty.
In view of the above background, it is an objective of the present invention to provide an improved echo cancellation circuit and method.